Pcbnew: fix Bug #1418135 (scrollbars never shown in design rules dialog, in netclasses grid list, which prevent to use many netclasses). Also fix very minor other issues.

* Using a Web navigator embedded in Kicad can create security issues in Kicad.
* The web kit is not always easily available on Linux.
* There is now a tool in kicad fp lib wizard  to list and select the .pretty libs available on Github Kicad repo (When the kicad plugin is selected, this tool can download and make a local copy of these libs)
However, using -D KICAD_USE_WEBKIT=ON when invoking  cmake still build the embedded web viewer.

Pcbnew - plot dialog: fix Bug #1417435 (Gerber(and some others)  plot mode is fixed as”sketch” instead of "filled" )

Dialogs edit footprint in module editor and edit footprint in board editor: add button to edit the selected 3D shape filename.
Dialog edit component in library editor: add button to edit the selected footprint filter.

Dialogs edit footprint in module editor and edit footprint in board editor: add button to edit the selected 3D shape filename.
Dialog edit component in library editor: add button to edit the selected footprint filter.

Pcbnew: simplify draw code in  default canvas and plot functions by removing the plot LINE option, which is not existing in all plotters, not existing in opengl/cairo canvas, and not usefull in default canvas.

ModEdit: add a settings menu option(and the corresponding dialog) to set default values used when creating a footprint or add graphic items to a footprint being edited.
These settings are now not common to the board editor, because the footprint editor can be run outside the board editor.
download_boost.cmake: ignore  .htm* files when installing boost. They are not used, and often have a very very long (and stupid) name which sometimes creates issues.

Class BOARD: Rename an overloaded method named SetLayer with the better name SetLayerDescr, and comment it. It also remove some warnings when compiled with option -Woverloaded-virtual

* Fix -Woverloaded-virtual warning in DIALOG_LIB_NEW_COMPONENT class
  definition.
* Make EDA_ITEM::Matches( const wxString& wxFindReplaceData& ) protected
  since it's a helper function that should only be called by derived objects.

Footprint Editor: New footprint creation: change defaults for ref and value texts: now Ref text is REF** and Value text  is the footprint name. Also Value is put on fab layer (front)
muonde.cpp: cleanup this very old code, and fix not working code.

* Check to see if image handler is already initialized to prevent warning
  on debug builds.
* Change LIB_PART destructor message output to use wxLogDebug for debugging
  on windows builds.
* Minor code cleaning and coding policy fixes.

Fix minor aesthetic issues in dialogs (these minor issues (overlapping widgets)  exit only with some Windows managers like KDE)

Pcbnew: fix Bug #1413701 (Segfault on highlight "unconnected" net 0 in Pcbnew ). Also, remove now useless code and other very minor fixes.

Pcbnew: fix issues and potential issues when reading board files and mainly old board files, and enhance the DRC tests and Delete Single Pad Net option when reading a netlist:
* Delete Single Pad Net option does not delete the net if a zone use this net (i.e. is attached to this pad).
* pcb_parser accept now negative netcodes in zones (can happen with old files, which previously could crash Pcbnew)
* pcb_parser accept now files with incorrect or missing net count  (can happen with old files, which previously could crash Pcbnew)
* if a zone has a non-existent net name it now keep this net name, and DRC detect it (previously, the net name was lost, and the DRC did not tected this issue).
* Drc test: now detect a "dead" net, i.e. a net with 0 pads, but still used by a zone. It happens easily after a schematic modification, when a net disappears or is renamed.

Eeschema: fix a minor bug and enhances SCH_MARHER::Matches function (  partial use of  0005-SCH_MARKER-fix-a-Woverloaded-virtual.patch, from <fatzer2@gmail.com> ) and other very minor fix.

Pcbnew, layers manager: Fix an annoying bug I added in rev 5359, in Render options: in Board editor many render options were not displayed (similar fp editor mode).
Now fixed.

Pcbnew: Fix potential bug in DRAG_SEGM_PICKER (a variable type bool was used as int. type is now int). Minor code cleanup.

Very minor fixes: add missing const return type to some GetBoundingBox() declarations. Change few functions names to better names in some places (like GetName() to GetPinName() )

Minor and very minors fixes: move -DKICAD_USE_WEBKIT definition to the main CMakeLists.txt. Remove duplicate semi-colon at end of some lines.